Seed Plants: Angiosperms

Outline

• Phylum Magnoliophyta Gametophyte Development

• Pollination• Fertilization and Seed Development• Specialization Trends in Flowering Plants• Pollination Ecology• Herbaria and Plant Preservation

Overview

• Angiosperms are plants with seeds contained within a vessel (carpel). Largest and most diverse phylum of the

Plant Kingdom.

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Phylum Magnoliophyta

• Flowering Plants Angiosperms are heterosporous.

- Female gametophytes are wholly enclosed within sporophyte tissue and reduced to only a few cells.

- At maturity, male gametophytes consist of a germinated pollen grain with three nuclei.

Phylum Magnoliophyta

• Development of Gametophyte Diploid megasporocyte cell differentiates

from all other cells in the ovule.- Undergoes meiosis and produces four

haploid megaspores. Three degenerate

Integuments differentiate and eventually become the seed coat.

- Leave micropyle at one end.

Gametophyte Development

• Central Cell Nuclei may become a binucleate cell, or may fuse together, forming a single diploid nucleus. One of the cells functions as the egg.

• Female gametophyte (megagametophyte) consists of a large sac usually containing eight nuclei in seven cells.

Mature Megasporocyte

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Gametophyte Development

• While megagametophyte is developing, a parallel process that leads to the formation of male gametophytes occurs in the anthers. Four patches of tissue differentiate from

the main cell mass.- Contain diploid microsporocyte cells

which undergo meiosis and produce a quartet of microspores.

Lily Anther Cross Section

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Gametophyte Development

• After meiosis, the haploid microspores in the pollen sac undergo several changes. Nucleus in each microspore divides by

mitosis. Members of each quartet of microspores

separate from one another. A two-layered wall develops around each

microspore.• Microspores are now pollen grains.

Outer wall - exine.

Pollination

• Pollination is the transfer of pollen grains from an anther to a stigma.

• Fertilization involves the union of an egg and a sperm. May not occur until days or weeks or even

months after pollination has taken place.

Fertilization and Seed Development

• After pollination, further development of the male gametophyte may not take place unless the pollen grain is: From a different plant of the same species. From a variety different from that of the

receiving flower.- Under suitable conditions, a pollen tube

grows down between the cells of the stigma and style until it reaches the ovule micropyle.

Fertilization and Seed Development

• When the pollen tube reaches the micropyle, it continues to the female gametophyte and discharges its contents. Double Fertilization (Fusion) Occurs

- One sperm migrates from the synergid to the egg and forms a zygote.

- Other sperm cell migrates from the synergid and unites with the central cell nuclei producing a triploid endosperm nucleus.

Mature Male Gametophyte

Apomixis and Parthenocarpy

• Apomixis is development or fusion of gametes but with the normal structures otherwise being involved. Fruits that develop from ovaries with

unfertilized eggs are parthenocarpic.- Seedless (Navel oranges)

Seedless watermelon are produced by crossing varieties with different numbers of chromosomes. (Not parthenocarpic)

- Fertilization and seed formation don’t occur.

Specialization Trends in Flowering Plants

• Fossil record suggests flowering plants first appeared about 160 million years ago during the late Jurassic period. Most botanists hypothesize primitive

flowers had numerous spirally-arranged parts that were not fused together and were variable in number.

- Flowers were regular and contained both stamens and pistils.

Specialization Trends in Flowering Plants

• In advanced flowers, the receptacle has fused to the ovary. When the ovary is embedded in the

receptacle, it is said to be inferior.- Flower parts attached to to the top of the

ovary are epigynous. When the ovary is produced on top of the

receptacle, it is said to be superior.- Flower parts attached around the base

are hypogynous.

Specialization Trends in Flowering Plants

• Flowers have tended to become irregular.• Complete flowers contain a calyx, corolla,

stamens, and a pistil. Perfect flowers have stamens and a pistil.

- Imperfect flowers are missing either stamens or a pistil.

• Monoecious plants have both male and female imperfect flowers. Dioecious plants only have only male or

female flowers.

Pollination Ecology

• Throughout the evolutionary history of flowering plants, the pollinators have evidently coevolved with plants. Twenty thousand bee species are included

among current-day pollinators.- Many bee-pollinated flowers are

delicately sweet and fragrant. Flowers pollinated by beetles tend to

have different, stronger odors.

Pollination Ecology

• Moth and butterfly-pollinated flowers also often have sweet fragrances. Night-flying moths tend to visit white or

yellow flowers. Butterflies tend to visit bright blue, yellow,

or orange flowers.- Nectaries are at bottom of corolla tubes.

Situated for specialized mouth parts.

Pollination Ecology

• Flowers visited by birds are often bright red or yellow, and usually have little, if any, odor. Typically large flowers.

• Birds are highly active pollinators and tend to burn energy rapidly. Many bird-pollinated flowers produce

copious amounts of nectar to assure repeated visits.

• Bats tend to visit flowers that open only at night.

Herbaria and Plant Preservation

• Herbaria are essentially libraries of dried, pressed plants, algae, and fungi, arranged and labeled. Properly prepared and maintained

specimens may remain in excellent condition for 300 or more years.

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Review

• Phylum Magnoliophyta Gametophyte Development

• Pollination• Fertilization and Seed Development• Specialization Trends in Flowering Plants• Pollination Ecology• Herbaria and Plant Preservation

Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display